Precapture of CO2 and Hydrogenation into Methanol on Heterogenized Ruthenium and Amine‐Rich Catalytic Systems

Abstract A heterogenized alternative to the homogeneous precapture of CO2 with amines and subsequent hydrogenation to MeOH was developed using aminated silica and a Ru‐MACHOTM catalyst. Commercial mesoporous silica was modified with three different amino‐silane monomers and used as support for the Ru catalyst. These composites were studied by TEM and solid‐state NMR spectroscopy before and after the catalytic reaction. These catalytic reactions were conducted at 155 °C at a H2 and CO2 pressures of 75 and 2 bar, respectively, with the heterogeneous system (gas‐solid) being probed with gas‐phase infrared spectroscopy used to quantify the resulting products. High turnover number (TON) values were observed for the samples aminated with secondary amines.


Introduction
Global warming is a problem that has been and is tackled by numerous policy approaches, technological inventions and behavioral changes. Most energy-system evaluations assume it is necessary for the massive amounts of industrially emitted CO 2 to be eliminated by means of carbon capture and storage. [1] However, alternative processes, in which the captured CO 2 is recycled back into fuels and materials, are actively researched. [2,3] Although converting captured CO 2 to useful molecules is appealing, the high stability of CO 2 means that its transformation requires some combination of high energy input, high temperatures and pressures, and the use of highly reactive compounds such as H 2 .
The production, storage and distribution of renewable H 2 is itself a challenging task; hence the extent to which it can be used to convert CO 2 will depend on economic factors such as market size, availability of renewable feedstock and environmental impact of the H 2 -generation technologies chosen. [4] H 2 can be generated from the electrolysis of H 2 O using renewable energy sources such as solar, wind, etc. [5] Many fuel-related C 1 products such as CH 4 , methanol (CH 3 OH), and HCOOH can be obtained by treating CO 2 with H 2 . [6][7][8] Among CO 2 hydrogenation products, CH 3 OH is one of the most attractive because it can be used as a drop-in liquid fuel for already-established internal combustion engines and direct methanol fuel cells (DMFC). [9,10] Moreover, CH 3 OH is a viable H 2 storage medium (12.5 wt % H 2 ) and a convenient chemical feedstock for a broad array of chemicals and products including ethylene and propylene (through the methanol-to-olefin process). [11] CH 3 OH is already one of the most important building blocks and solvents in the chemical industry, with an annual production of 98 million tons in 2019. [12] Industrially CH 3 OH is currently synthesized from syngas (CO, H 2 , and CO 2 ) over Cu/ZnO/Al 2 O 3 -type heterogeneous catalysts under high pressures (> 60 atm) and elevated temperatures (> 200°C); [13][14][15] however, CO 2 has also been converted to CH 3 OH using homogeneous catalysts. [16] The most effective discrete catalysts include ruthenium-based pincer complexes, [17][18][19][20] but other complexes are also promising. [21] A precapture of the CO 2 in the form of carbamates, bicarbonates, and maybe carbamic acids with amines or other bases and two-phase systems enhances the overall conversion. [22][23][24][25] This can be achieved both in homogeneous and heterogeneous systems. In contrast to homogeneous catalysts, heterogeneous catalysts play an important role in industrial chemical production. [26] They are preferred because of their robustness and lower operational cost, in particular through their easier recovery/separation from the products allowing chemical processes to be streamlined.
The precapture approach in homogenous Ru-based systems [17][18][19][20] prompted us to hypothesize, prepare and test a heterogeneous Ru-based catalytic system that also contained tethered amines, which jointly enabled the hydrogenation of CO 2 to CH 3 OH. Our catalyst systems were built by grafting amines onto commercial mesoporous silica, then loading a homogeneous ruthenium catalyst (Ru-MACHO TM ), as illustrated in Figure 1. Solid adsorbents used as part of CO 2 hydrogenation have previously been reported by Prakash et al. [27]

Results and Discussion
The composites were prepared by first grafting one of three aminosilanes, namely (3-aminopropyl)triethoxysilane (APTS), (3-(methylamino)propyl)trimethoxysilane (APTMS(Me)) or (3-(2aminoethylamino)propyltriethoxysilane (AEAPTS), on chromatographic silica. Then, the Ru-MACHO TM catalyst was deposited on the modified silica to produce a composite that was tested in a custom reactor for the conversion of CO 2 into CH 3 OH under high H 2 pressure (75 bar of H 2 and 2 bar of CO 2 ) at a relatively low temperature of 155°C. CO 2 adsorption isotherms of the catalysts, measured at 0°C ( Figure S2, Supporting Information), showed that all the samples took up approximately 1 mmol of CO 2 /g of material. This is a gas-solid system, so the volatile products were analyzed using dispersive gas-phase IR spectroscopy, which is a non-destructive technique that can quantify multiple components across a relatively broad concentration range with a single measurement, and can provide detection limits down to the parts-per-billion (ppb) range for many components. This technique is especially useful here because of the presence of CO in the gas mixture, as it can be difficult to detect and quantify using mass spectrometry, as the m/z = 28 fragment could come from CO 2 or from the carrier gas used). [28] The CO 2 conversion and the TON reached 32 % and 3800 for Ru_AEAPTS (Table 1). Here, the TON denotes the amount of MeOH formed in relation to the amount of catalyst (the equation used is shown in the Supporting Information). Thus Ru_AEAPTS converted almost three times as much CO 2 to MeOH as Pd nanoparticles on monoaminated silica, [29] although Ru_AEAPTS_2 (with half the loading of immobilized molecular catalyst) converted less CO 2 than either of these systems. CO 2 that has been immobilized on secondary rather than primary amines is hydrogenated to CH 3 OH with better selectivity over Ru-based homogenous catalysts, [25] consistent with our findings that aminated supports bearing secondary amines were not only more selective but also more active than Ru_APTS. Computational studies of homogenous catalytic systems have established that the CÀ N breakage in dimethylaminomethanol was more energetically favored than in methylaminomethanol, meaning that secondary amines capture CO 2 less strongly, and could be preferred over primary amines in the precapture-andhydrogenation cycle of CO 2 . [25] The conversion of CO 2 into CH 3 OH on aminated silica with a heterogenized molecular catalyst is a complex multistage process that requires both catalyst components; neither silica modified with APTS(Me) but without Ru-MACHO TM , nor Ru-MACHO TM supported on unmodified silica catalyzed the hydrogenation of CO 2 . Precapture of CO 2 on aminated supports appears to play an important part in its conversion into CH 3 OH and secondary amines were advantageous in terms of overall activity. [30] In addition to the desired MeOH, two byproducts were observed from the hydrogenation: CO, which can be formed through the reverse water-gas shift reaction [31] and ammonia, which may have formed from the decomposition of the amine groups on the aminated support. We observed a competition between the hydrogenation of CO 2 and the reverse water-gas shift reaction, as is usual in these types of systems, [32] and the product distribution depends on both the amine that was grafted on the support and the loading of Ru. This variation suggests that the interface between the adsorbed species and the active center is important in the control of the selectivity of  [a] The reactor was first purged using a Schlenk line, then sealed and loaded with the appropriate amount of catalyst inside a glovebox. Once loaded, the reactor was submitted to 2 bar CO 2 for 2 h at 25°C. The pressure was then switched to 75 bar H 2 and the temperature was increased to 155°C. The reactor was held at that temperature for 40 h, then cooled to room temperatures and the gases were extracted with vacuum and N 2 flushing into a gas-phase IR cell and analyzed (see Supporting Information for details).
[b] The conversion is calculated in relation to the 2 bar of initial CO 2 .
the catalysts, in agreement with the recently reported findings of Pazdera et al. regarding the use of Pd nanoparticles on silica aminated with primary amines for the hydrogenation of CO 2 to CH 3 OH. [29] They concluded that the activity of their catalyst was given by the interface between Pd nanoparticles and the aminated support, and that a higher concentration of active centers led to a distribution of products leaning towards the formation of CH 3 OH over CO. In our study, it was clear that secondary amines were much more advantageous than primary amines to the hydrogenation of precaptured CO 2 to CH 3 OH. Both the catalytic systems in this study and those of Pazdera et al. [29] focusing on Pd nanoparticles and monoaminated catalytic systems can catalyze the synthesis of CH 3 OH at temperatures that are significantly lower than for the commercially used catalysts. [13][14][15]33] Clearly, the amine-modified silica support plays a key role in the selectivity and activity of the catalyst, and secondary amines are preferred for the precapture of CO 2 . In our case, the aminated supports with secondary amines, that is, Ru_APTS(Me) and the diaminated support (Ru_ AEAPTS), had higher TON, while the one with exclusively primary amines (Ru_APTS) had a lower TON. The diaminated system and the secondary amines also had a significantly higher conversion of CO 2 into CH 3 OH or CO than the monaminated one. For Ru_AEAPTS, which showed the highest TON, we studied two different loadings of catalysts. Ru_AEAPTS_2, with the lower loading of supported molecular catalyst, gave a lower TON, but a higher selectivity to CO over CH 3 OH. Besides CH 3 OH, CO and ammonia were observed in the gasphase IR spectra ( Figure S5) used for the quantification, highlighted in Table 1. Diamines that are grafted on solid supports are less stable than their monoamine counterparts. [34] This tendency could explain why more ammonia was generated from Ru_AEAPTS under the reaction conditions; however, the monoaminated support with secondary amines, Ru_APTS(Me), also yielded a high concentration of ammonia, which could limit its reusability (without any regeneration steps). Thermogravimetric analysis (Table S1) indicated that the amount of aminosilane fragments present during catalysis was approximately two orders of magnitude higher than the detected ammonia.
In order to try and understand the nature of the immobilized molecular catalyst, transmission electron microscopy images and solid-state NMR spectra were recorded for the bestperforming composite (Ru_APTS(Me)) before and after hydrogenation of CO 2 . This was done to assess if there were any notable morphological or chemical changes in the composites after the reactions. As such, analysis of the TEM images for fresh (Figure 2a,b) and post-catalysis (Figure 2c,d) Ru_APTS(Me) showed that the ruthenium species were evenly distributed on the silica support, and no agglomeration of ruthenium could be seen, which can be also observed in the EDS mapping ( Figure S7). Interestingly, the reaction conditions do not change the apparent nature of the composite as the used samples present the same morphology under the TEM microscope.
Solid-state 1 H MAS as well as 1 H-13 C CPMAS NMR spectra of the Ru_APTS(Me) catalyst were collected to assess the chemical nature and the integrity of the involved species. 1 H MAS NMR spectra of the unused (Figure 3a, black trace) as well as the post-reaction catalyst (Figure 3a, red trace) displayed two distinct groups of overlapping resonances. Signals in the chemical shift range from 0 to 4 ppm originated from protons in alkyl, methyl, and amine groups of Ru_MACHO TM and APTS(Me). [35] Signals between 6 and 10 ppm originated from the aromatic ligands of the Ru-MACHO TM catalyst (see Figure 3c for molecular structure). Interestingly, in the same sample, an additional 1 H signal at around À 17 ppm is observed. This signal could be attributed to the activation of the Ru center after the hydrogenation leading to the formation of a hydride. [36] In the 13 C NMR spectra shown in Figure 3b, several distinct resonances are observed. To provide assignments of these signals, 13 C NMR shifts were calculated for the models of Ru_ MACHO TM and APTS(Me) at the DFT level using the previously validated protocol of Jaworski and Hedin, [37] and results are shown in Figure 3d,e. 13 C NMR signals at 10, 22, 35, and 54 ppm can be unambiguously assigned to the alkyl and methyl groups of the APTS(Me) (Figure 3e). Signals of alkyl carbon atoms from the Ru_MACHO TM are observed at 51 ppm, and overlap with the signal at 38 ppm. Aromatic ligands of the Ru complex are responsible for a distinct group of resonances centered at 132 ppm. Based on our previous studies of amine-modified silicas, [35] signals between 150 and 200 ppm are most probably due to the interactions of the aminated support with CO 2 and H 2 O. Signals at 63 and 73 ppm do not originate from the catalyst, and most probably are due to the presence of alcohol and methoxy groups in the sample from unreacted chains from the silanols used for the grafting, with the broadening in the post-reaction samples attributed to the presence of different configurations of methanol that formed during the reaction and remained bound. In analogy to the 1 H NMR data of Figure 3a, 13 C NMR spectra reveal a relative increase of the aromatic signals compared to the alkyl/methyl spectral region.

Conclusion
In conclusion, we highlight a heterogenized catalytic system consisting of aminated silica supports and the molecular catalyst, Ru-MACHO TM , that is active in the hydrogenation of CO 2 to CH 3 OH at a relatively low temperature of 155°C. The most remarkable finding was the high TON values for the aminated catalytic systems containing secondary amines. The temperature used for the CO 2 conversion in this study was significantly lower than for regular heterogeneous solid catalysts. [33] Our catalytic conversion was almost three times higher than that observed by Pazdera et al. [29] for the catalytic transformation of CO 2 on Pd nanoparticles on monoaminated silica, while diaminated derivatives had not been studied. Notably, the product analysis in the present study was carried out using a gas-phase IR analysis that allows the complete identification and quantification of the different species. We foresee that further experimentation and optimization of the reaction conditions for the systems investigated here could enable lower reaction temperatures and pressures, as well as allowing to study the reusability and long-term stability of the system. When compared to other applied heterogeneous catalytic systems, [33] our highest achieved conversion of 33 % is lower than the 66 % reached by using Cu-ZnO/Al 2 O 3 . [38] Our system, however, was able to convert CO 2 to methanol at lower temperatures, that is, 155°C instead of 260°C. In addition, more in-depth studies of the reaction mechanism with in situ knowledge of what is occurring during the reactions could help to understand the detailed chemistry. Such understanding could enable the design of the catalytic system to be more practical and applicable for industrial purposes.
The amine-rich support was prepared using the methodology reported previously. [35,39] In summary, for each synthesis, 3 g of dried porous silica and 180 cm 3 of toluene were added to a threenecked flask equipped with a Dean-Stark reflux condenser. This was heated to 50°C under stirring for 30 min; 0.3 mL of H 2 O was added and the mixture was refluxed for 1 h. After this period had elapsed, the required amount of silane monomer (APTS, APTS(Me) or AEAPTS; based on previous studies, [40] five mol of silane per mol of free OH in the substrate) was added and the mixture was left to reflux for 24 h. The solid was filtered off and extracted in a Soxhlet extractor for 16 h with fresh toluene as solvent to ensure the removal of unreacted silanes. Finally, the solid was washed with toluene (2 × 50 cm 3 ) and ethanol (3 × 50 cm 3 ) and dried overnight at 110°C. Following this grafting procedure, the amine-modified silica was introduced into a glovebox, where it was added to a solution of Ru-MACHO TM catalyst in the minimum amount of 1,4-dioxane to dissolve the Ru complex. The amount of solution was chosen to give m(Ru-MACHO TM )/m(aminated silica + Ru-MACHO TM ) = 0.05 (or 0.025 in the case of Ru_AEAPTS_2). The solution and modified support were stirred together inside the glovebox until the solvent had completely evaporated. The resulting pale yellow solid was stored in a vial in the glovebox until use.

Thermogravimetric analysis
Thermogravimetric analysis (TGA) was used to record the mass loss on heating Silica_APTS, Silica_APTS(Me) and Silica_AEAPTS using a TA Instruments Discovery (TA Instruments, Stockholm, Sweden) thermobalance in dry air, for which samples were heated from 50 to 950°C at a rate of 10°C min À 1 in a platinum cup.

CO 2 -adsorption measurements
Adsorption isotherms of CO 2 on Ru_APTS, Ru_APTS(Me) and Ru_ AEAPTS were measured at 0°C using a Micrometrics ASAP2020 volumetric adsorption analyzer. Sample tubes were loaded in a glovebox under an inert N 2 atmosphere, then pretreated under high dynamic vacuum conditions at 110°C for 10 h. During measurement, thermal control was achieved by immersing the samples in a Dewar flask filled with~3 dm 3 of H 2 O and ice, which equilibrated the temperature to 0°C.

CO 2 hydrogenation
The reactions were carried out in a homemade fixed bed batch reactor ( Figure S3). In a glovebox, 100 mg of catalyst was loaded in the reactor. The reactor was then flushed and pressurized to 2 bar CO 2 and allowed to equilibrate for 2 h. Following this, the gas was switched to H 2 and the pressure was increased to 75 bar and the temperature raised to 155°C and held at that temperature for 40 h, then cooled to RT. In addition, the reaction was tested using a sample that consisted of APTS(Me) modified silica (without the presence of the Ru species) as well as the Ru species immobilized on unmodified silica.

IR analysis of the gas phase
IR gas analysis was used to quantify the gas products. After the reaction, the cooled reactor was reheated to 100°C and connected to the infrared gas cell. The cell was flushed with dry N 2 and evacuated under a dynamic vacuum and the gas products from the reactor were subsequently collected into the cell for analysis.
The spectra were measured in the region of 600-4000 cm À 1 with a spectral resolution of 0.5 cm À 1 . For quantification, a collection of quantitative gas-phase IR spectra was used (QAsoft, Infrared Analysis Inc.). Reference spectra of known concentrations of certain compounds were spectrally subtracted from the measured spectra and the concentrations were calculated using the subtraction factor. [41] Bands (or part of the bands) with intensities below 0.1 absorbance units were chosen for analysis in order to work in the linear region of the Lambert-Beer law. Figure S4 contains reference spectra of the various compounds.

Transmission electron microscopy (TEM)
TEM images were taken on a JEOL JEM-2100F transmission electron microscope, equipped with a Schottky-type field emission gun operating at an accelerating voltage of 200 kV.

Solid state nuclear magnetic resonance (ss NMR)
1 H magic-angle spinning (MAS) NMR experiments were performed at a magnetic field of 14.1 T ( 1 H Larmor frequency of 600.12 MHz) on a Bruker Avance-III spectrometer with a 1.3 mm probehead and 60.00 kHz MAS rate. Acquisitions involved a rotor-synchronized, double-adiabatic spin-echo sequence with a 90°excitation pulse of 1.25 μs followed by a pair of 50.0 μs tanh/tan short high-power adiabatic pulses (SHAPs) with a 5 MHz frequency sweep. All pulses operated at a nutation frequency of 200 kHz. 128 signal transients with a 5 s relaxation delay were collected. 1 HÀ 13 C cross-polarization (CP) MAS experiments were performed at a magnetic field of 9.4 T (Larmor frequencies of 400.13, and 100.61 MHz for 1 H and 13 C, respectively) on a Bruker Avance-III spectrometer with a 4 mm probehead and 14.00 kHz MAS rate. Experiments involved Hartmann-Hahn-matched 1 H and 13 C radiofrequency fields applied for 1.5 ms, SPINAL-64 proton decoupling and 5 s relaxation delay. Chemical shifts were referenced with respect to neat tetramethylsilane (TMS). Models of the Ru_MACHO TM and APTS(Me) were energy optimized at the revPBE-D4/def2-TZVP(Ru)/pcseg-1 level of theory, and the 13 C NMR shifts subsequently calculated at the PBE0/ def2-TZVP(Ru)/pcSseg-1 level with the GIAO approach; in analogy to the protocol in Ref. [37]. Calculations were done with the ORCA code. [42]

Supporting Information Summary
The Supporting Information contains TG curves, CO 2 adsorption isotherms, details on the reactor and IR spectra from the reactions. Additional TEM images and EDS analysis is also included as well as data for the electronic structure calculations.